The subject invention is directed generally to unitized multilayer circuit structures, and more particularly to a unitized multilayer circuit structure that includes mechanically detectable alignment features that provide reference locations for automated placement of circuit devices in the unitized multilayer circuit structure.
Hybrid multilayer circuit structures, also known as hybrid microcircuits, implement the interconnection and packaging of discrete circuit devices, and generally include a unitized multilayer circuit structure formed from a plurality of integrally fused insulating layers (e.g., ceramic layers) having conductor traces disposed therebetween. The discrete circuit devices (e.g., integrated circuits) are commonly mounted on the top insulating layer so as not to be covered by another insulating layer or on a insulating layer having die cutouts formed thereon to provide cavities for the discrete devices. Passive components such as capacitors and resistors can be formed on the same layer that supports the discrete devices, for example, by thick film processes, or they can be formed between the insulating layers, for example, also by thick film processes. Electrical interconnection of the conductors and components on the different layers is achieved with vias or holes appropriately located and formed in the insulating layers and filled with conductive material, whereby the conductive material is in contact with predetermined conductive traces between the layers that extend over or under the vias.
Unitized multilayer circuit structures are commonly made pursuant to co-fired technology wherein a unitized multilayer circuit module is made from layers of pyrolyzable dielectric tape (comprising for example a ceramic material) known in the art as "green tape". Generally, each of the green tape layers of a particular module is punched and screen printed to include a predetermined pattern of conductive vias, interconnecting conductive traces, and electrical components such as resistors and capacitors, or portions of capacitors. The individual screen printed green tape layers for a module are then stacked in the required order, and laminated together using a chosen temperature and pressure. The laminated structure is then fired at an elevated temperature. Such co-fired technology has been implemented with low temperature co-fired ceramic (LTCC) as well as with high temperature co-fired ceramic (HTCC).
Examples of low temperature co-fired processing can be found in "Development of a Low Temperature Co-fired Multilayer Ceramic Technology," by William A. Vitriol et al., 1983 ISHM Proceedings, pages 593-598; "Processing and Reliability of Resistors Incorporated Within Low Temperature Co-fired Ceramic Structures," by Ramona G. Pond et al., 1986 ISHM Proceedings, pages 461-472; and "Low Temperature Co-Fireable Ceramics with Co-Fired Resistors," by H. T. Sawhill et al., 1986 ISHM Proceedings, pages 268-271.
After a multilayer circuit structure is assembled and fired, circuit devices are placed on the multilayer circuit structure and secured thereto, for example on the top layer thereof or on the exposed surfaces of one or more cavities formed in the top portion of the multilayer circuit structure. The tolerance of shrinkage of LTCC during firing results in unpredictable substrate dimensions which in turn makes accurate placement of the circuit devices difficult. Such device placement is currently being performed with an optical recognition system that keys off alignment marks printed on the particular exposed surface on which a circuit device is to be placed. Although fast and accurate, optical recognition device placement systems are expensive.